This application is related to Applicants"" patent application titled xe2x80x9cProcess For The Production Of Highly Branched Fischer-Tropsch Products And Potassium Promoted Iron Catalystxe2x80x9d having the same filing date, the entire contents of which are incorporated herein by reference.
The present invention relates to a process for increasing the yield of olefinic products, especially those olefins boiling within the range of diesel, from a slurry-type Fischer-Tropsch unit.
The market for lubricating base oils of high paraffinicity is continuing to grow due to the high viscosity index, oxidation stability, and low volatility relative to viscosity of these molecules. The products produced from the Fischer-Tropsch process contain a high proportion of wax which make them ideal candidates for processing into lube base stocks. Accordingly, The hydrocarbon products recovered from the Fischer-Tropsch process have been proposed as feedstocks for preparing high quality lube base oils. See, for example, U.S. Pat. No. 6,080,301 which describes a premium lube base oil having a high non-cyclic isoparaffin content prepared from Fischer-Tropsch waxes by hydroisomerization dewaxing and solvent dewaxing.
As discussed in U.S. Pat. No. 6,090,989, branching present on the molecule has a significant impact on the properties of the lube base stock. The type and position of branching present in the carbon backbone of the molecules is important in determining the properties of the lube base oil product. See S. J. Miller, Wax Isomerization for Improved lube Oil Quality, 1 st Intl. Conf. on Refining Processing, AICLE, New Orleans (1998). The ability to increase the molecular branching is not only advantageous for improving the qualities of lube base oils, but also benefits the lighter cuts derived from the Fischer-Tropsch product. For example, branching in Fischer-Tropsch derived jet will improve the freeze point, and branching in Fischer-Tropsch derived diesel is known to improve the pour point. See U.S. Pat. No. 5,506,272. For those Fischer-Tropsch products intended as feed for a hydrocracking operation a further advantage is that the branching renders the molecule easier to crack.
One method for introducing branching into Fischer-Tropsch-derived products is to oligomerize the olefins which are present in the product stream leaving the Fischer-Tropsch reactor. The oligomerization of alpha olefins introduces methyl branching into the carbon backbone. As already noted branching results in desirable lubricating properties. U.S. Pat. No. 4,171,320 teaches a Fisher Tropsch synthesis using a ruthenium catalyst to preferentially produce C2-C5 olefins. U.S. Pat. No. 4,417,088 describes a process for oligomerizing olefins to produce molecules having desirable branching.
The high temperature Fischer-Tropsch process which is carried out in the vapor phase usually will produce lower molecular weight olefinic products within the C3 to C8 range. The olefinic products of the high temperature Fischer-Tropsch process may be sent through oligomerization and hydrogenation steps which will produce a highly branched iso-paraffinic product. A disadvantage of the high temperature Fischer-Tropsch process is the presence in the product of significant amounts of aromatics. If the olefins recovered from the high temperature Fischer-Tropsch products are oligomerized to produce higher boiling range products, these aromatics will be left behind in the unreacted fraction, and their presence will diminish the quality of the lube and fuels. In contrast, the low temperature Fischer-Tropsch process which is conducted in liquid phase will yield higher molecular weight products characterized by low branching, less olefinicity than the high temperature Fischer-Tropsch process, and virtually no aromatics. While the low temperature Fischer-Tropsch process will produce products within the lube base oil boiling range, due to the low level of branching, such products do not possess the desired pour point characteristics. In order to meet these desired values for the products, a catalytic dewaxing operation is usually necessary in order to introduce the proper branching into the molecule. The relatively severe conditions at which the catalytic dewaxing unit must be operated results in a significant yield loss of the higher molecular weight products due to wax cracking. In addition, since the products derived from a low temperature Fischer-Tropsch operation have low olefinicity, a dehydrogenation step is necessary if an oligomerization operation is to be used to increase the yield of higher molecular weight products and introduce additional branching into the products. While a Fischer-Tropsch reaction may be suitably conducted in either a fixed bed reactor, slurry bed reactor, or a fluidized bed reactor, fixed bed reactors and slurry bed reactors are preferred for low temperature Fischer-Tropsch processes. Fluidized bed reactors are preferred for high temperature Fischer-Tropsch processes. Although the low temperature Fischer-Tropsch process is generally considered as being carried out at a temperature between 160 degrees C. and 250 degrees C. while the high temperature Fischer-Tropsch process is usually conducted at temperatures between 250 degrees C. and 375 degrees C., in actuality, the temperature range for the two processes will overlap. A good comparison of the high temperature and low temperature Fischer-Tropsch processes is presented in B. Jager and R. Espinoza, Advances in Low Temperature Fischer-Tropsch Synthesis, Catalysts Today 23 (1995) pp. 17-28.
Precipitated iron catalysts promoted with potassium have been described in the literature for use in Fischer-Tropsch synthesis. However, U.S. Pat. No. 4,994,428 teaches that the amount of potassium present should be limited to less than 0.6 weight percent. Higher levels of potassium are taught to offer no benefit in selectivity and to increase the production of undesirable oxygenated by-products. Copper is known to serve as an induction promoter, i.e., reduce the catalytic induction period, in a slurry-type potassium promoted iron catalyst. Copper and potassium promoted iron catalysts have been described in the literature as being selective for alpha olefins. See U.S. Pat. No. 5,100,856. U.S. Pat. No. 4,639,431 describes an iron/zinc Fischer-Tropsch catalyst promoter with copper which is useful for producing olefins.
The present invention utilizes a novel iron-based Fischer-Tropsch catalyst promoted with very high levels of potassium which allow the Fischer-Tropsch unit to produce products having high olefinicity, i.e. a high percent of the total hydrocarbon products will contain double bonds. The present invention is particularly useful for producing high yields of olefins boiling within the range of diesel. In addition, by varying the amount of potassium promoter it is also possible to make higher molecular weight products with increased branching as compared to the conventional low temperature Fischer-Tropsch process. A further advantage of the present invention over conventional high temperature Fischer-Tropsch processes is that the products contain very low levels of aromatics. This invention makes it possible to design an integrated process which maximizes the yield of high value lube base oil or, if desired, maximize the production of high quality transportation fuels, such as diesel and jet. As such, it combines the best features of both the low temperature and high temperature Fischer-Tropsch processes and offers greater flexibility for the plant design and product slate than has hitherto been possible.
As used in this disclosure the words xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase xe2x80x9cconsists essentially ofxe2x80x9d or xe2x80x9cconsisting essentially ofxe2x80x9d is intended to mean the exclusion of other elements of any essential significance to the composition. The phrases xe2x80x9cconsisting ofxe2x80x9d or xe2x80x9cconsists ofxe2x80x9d are intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.
In its broadest aspect, the present invention is directed to a method for increasing the olefinicity of a C5 to C19 Fischer-Tropsch product which comprises (a) contacting a synthesis gas feed stock in a slurry-type Fischer-Tropsch reaction zone with a potassium promoted iron catalyst under slurry-type Fischer-Tropsch reaction conditions, wherein the atomic ratio of iron to potassium in the catalyst is within the range of from about 100 atoms of iron to atoms of potassium to about 100 atoms of iron to 15 atoms of potassium and (b) recovering from the Fischer-Tropsch reaction zone a C5 to C19 Fischer-Tropsch product having an olefinicity of at least about 60 weight percent. The present invention is further directed to a process for increasing the average molecular weight of the product from a Fischer-Tropsch unit comprising (a) contacting a synthesis gas feed stock in a slurry-type Fischer-Tropsch reaction zone with a potassium promoted iron catalyst under slurry-type Fischer-Tropsch reaction conditions, wherein the atomic ratio of iron to potassium in the catalyst is within the range of from about 100 atoms of iron to 3 atoms of potassium to about 100 atoms of iron to 15 atoms of potassium; (b) recovering from the Fischer-Tropsch reaction zone a C5 to C19 Fischer-Tropsch product having an olefinicity of at least about 60 weight percent; and (c) oligomerizing under oligomerization conditions at least a portion of the C5 to C19 Fischer-Tropsch product and recovering an oligomerization product from the olgomerization reaction zone having an average molecular weight that is at least 10 percent higher than the C5 to C19 Fischer-Tropsch product. The present invention is particularly useful for producing highly olefinic products boiling in the range of naphtha and diesel, and most especially within the boiling range of diesel. Preferably the oligomerization product recovered in step (c) will be a C20 plus hydrocarbon.
As used in this disclosure, the term xe2x80x9cC19 minus Fischer-Tropsch productxe2x80x9d refers to a product recovered from a Fischer-Tropsch reaction zone which is predominantly comprised of hydrocarbons having 19 carbon atoms or less in the molecular backbone. One skilled in the art will recognize that such products may actually contain a significant amount of hydrocarbons containing greater than 19 carbon atoms. In general, what is referred to are those hydrocarbons having a boiling range of diesel and below. In general, for the purposes of this disclosure, diesel is considered as having a upper boiling point of about 700 degrees F. (370 degrees C.) and an initial boiling point of about 300 degrees F. (about 150 degrees C.). Diesel may also be referred to as C10 to C19 hydrocarbons. Likewise, the term xe2x80x9cC20 plus productxe2x80x9d refers to a product comprising primarily hydrocarbons having 20 carbon atoms or more in the backbone of the molecule and having an initial boiling point at the upper end of the boiling range for diesel, i.e., above about 650 degrees F. (340 degrees C.). Such products are often referred to as residuum, which includes both vacuum residuum and atmospheric residuum, since such products will typically make up the bottoms from the distillation column. It should be noted that the upper end of the boiling range for diesel and the lower end of the boiling range for residuum have considerable overlap. The term xe2x80x9cnaphthaxe2x80x9d when used in this disclosure refers to a liquid product having between about C5 to about C9 carbon atoms in the backbone and will have a boiling range generally below that of diesel but wherein the upper end of the boiling range will overlap that of the initial boiling point of diesel. The term xe2x80x9cC5 to C19 Fischer-Tropsch productxe2x80x9d refers to those products boiling within the range of naphtha and diesel. Products recovered from the Fischer synthesis which are normally in the gaseous phase at ambient temperature are referred to as C4 minus product in this disclosure. The precise cut-point selected for each of the products in carrying out the distillation operation will be determined by the product specifications and yields desired.
In order to maximize the production of olefins in the Fischer-Tropsch reactor the atomic ratio of potassium to iron in the Fischer-Tropsch catalyst will preferably fall within the range of from about 100 atoms of iron to 3 atoms of potassium and about 100 atoms of iron to 7 atoms of potassium. In addition, the Fischer-Tropsch catalyst will preferably contain from about 0.1 to about 3 atoms of copper per 100 atoms of iron.